^Concerning Spinda*, I don't think the effort to create all the patterns was that much bigger than the others. It's just a little randomizer script put into the code for the character model. The same goes for shiny values, changing the relevant colours is a matter of a couple lines of code. Rematch teams for the Elite Four would be a relatively trivial matter; there are more than a thousand scripted trainers in the game, and making five more wouldn't take more than a lunch break at most.
Also, I think I have a better reason that "coz thats kool": Doing it the current way means that you have more incentive to go to restaurants and battle the waiters there, than actually revisiting the so-called most powerful trainers in the region. The Elite Four are meant to be tough, powerful, epic and all that, but their impact is somewhat diminished by being easier to beat than the crew of a small restaurant.
Then again, I have long suspected that the Elite Four are kind of vain, and that nobody in the region really care much about them. Most of them never lift a finger to stop the regional evil team, they have their giant, flashy, empty castle in the middle of absolutely nowhere, with nobody visiting. Even their Pokémon Centre is almost deserted. Also, nobody ever bother to show up to see the child-prodigy, movie-star, mall-owner, world-saving, fashionist, contest-winner, Pokédex-filler, banisher of evil, speaker to everybody, gym-beating main character when (s)he's up to battle the most powerful trainers in the land. Even in the regions with media offices, they don't even send a TV team to cover the battles. You're supposed to be a great celebrity at this point, but when you fight the Elite Four there isn't even a spectator in sight. The PWT, however, seems to draw great crowds with an announcer, confetti, spotlights and the like.
Then, in Emerald and DPPt, you get to visit an entirely new place after beating the Elite Four. Heck, beating the Elite Four seems to be a prerequisite to get to visit some of those places. That implies that all the trainers present have beaten the Elite Four before you, and nobody even as much as mention it. Beating the Elite Four must be kind of like getting a drivers' license: You think it's awesome if you make it, you feel kind of special when you've passed the test, it opens lots of new possibilities, then you realize that millions of people have gone through it before without making a fuzz about it. There are probably tens of thousands of names engraved next to yours in the Hall of Fame.
*The plural of "Pokémon" is still "Pokémon", and that goes for species names as well. One Pikachu, the Pikachu, many Pikachu, all the Pikachu. Kind of like sheep. A minor annoyance of mine is that people still haven't learnt that after six generations of Pokémon.